Throughout this application, various patents are referred to by an identifying citation. The disclosures of the patents referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
There is a rapidly growing worldwide need for renewable energy sources to replace our dependence on energy from fossil fuels which are becoming increasingly expensive and unstable in supply and which produce carbon dioxide emissions that promote global warming. Among the various renewable energy sources, photovoltaic or solar cells are particularly attractive because of their potential to capture the extremely large amounts of energy from the sun and convert it to electricity to meet much of the world's need for energy. Also, photovoltaic energy sources do not emit carbon dioxide and thus help to abate the global warming trend.
Organic photovoltaic cells are one of the options to supplement or replace silicon and other inorganic photovoltaic cells that are relatively expensive and generally are limited to rigid, high cost designs. One of the general features of organic photovoltaic cells is their lower cost due to the ability to coat them economically on flexible substrates and their resulting flexible designs that will be easier and less expensive to integrate into buildings, clothing, and other substrates where energy capture, storage, and usage are desirable. For general commercial acceptance based on cost and other factors, the threshold value of solar energy conversion efficiency for organic photovoltaics is estimated to be about 8%, compared to about 15% for silicon photovoltaics.
Various technical approaches have been taken to make organic photovoltaic cells, such as for example illustrated by U.S. Pat. No. 5,171,373 to Hebard et al.; U.S. Pat. No. 5,331,183 to Sariciftci et al.; U.S. Pat. No. 5,470,505 to Smith et al.; U.S. Pat. No. and 5,885,368 to Lupo et al. These approaches generally involve at least one active layer that comprises a photon-absorbing organic compound that acts as an electron donor upon the absorption of a photon and leads to the flow of electrical current in the photovoltaic cell. Typically, these photon-absorbing organic compounds absorb in the ultraviolet and visible region of the sun's spectrum, but do not significantly absorb in the infrared portion of the sun's spectrum.
Since about 50% of the sun's energy is in the infrared part of the solar spectrum, it would be advantageous if an organic photovoltaic cell were available that would convert infrared photons to electricity